Iron oxide pigments which contain iron at the +2 oxidation stage are thermodynamically unstable phases compared with iron(III) oxide Fe.sub.2 O.sub.3. They may be partly or completely oxidized in the presence of air or oxygen, e.g. as follows: EQU 2Fe.sub.3 O.sub.4 +1/2O.sub.2 .fwdarw.3Fe.sub.2 O.sub.3.
Such reactions are known to take place, for example, in iron oxide black pigments which corresponds to magnetite in their composition and structure. Oxidation deprives the pigment of its most important property, namely its color and the pigment therefore becomes unusable. The tendency to oxidation naturally increases with the degree of fineness of subdivision and hence specific surface area of the pigments.
The same applies to mixtures of iron oxide black and other iron oxide color pigments such as iron oxide red or iron oxide yellow, which are produced for brown color shades.
Whereas in color pigments it is the loss of color properties which renders them unusable, in magnetic pigments containing iron (II) it is the loss of magnetic properties, which is also caused by oxidation. Particularly at risk are the finely divided magnetite pigments and mixed phases of magnetite (Fe.sub.3 O.sub.4) and maghaemite (.gamma.-Fe.sub.2 O.sub.3) which have a high iron(II)content. Mixed phases of magnetite and maghaemite and/or ferrites such as cobalt ferrites and magnetic pigments which are composed of a core of Fe.sub.3 O.sub.4 or an iron oxide in a state of oxidation between that of Fe.sub.3 O.sub.4 and .gamma.-Fe.sub.2 O.sub.3 and a sheath of magnetic metal oxides surrounding this core, in particular a sheath of iron and cobalt, are also sensitive to oxidation. In the technical literature on this subject, the term "bertholloids"is found for the compositions described here as well as the term "mixed phase".
It already been attempted to reduce the sensitivity to oxidation of finely divided ferrimagnetic magnetite pigments by a treatment with heterocyclic organic compounds (German 2,744,598).
After after-treatment with compounds containing boron is provided in German 3,211,327.
One disadvantage of such after-treated pigments is that they are frequently incompatible with the binders conventionally used.
It is therefore an object of the present invention to provide a way of improving the thermal stability of iron oxide pigments containing iron(II) in the presence of air or other gases containing oxygen or oxygen itself. At the same time, the pigment should also be free from the other disadvantages of pimgents known in the art.